1. Field of the Invention
The present invention relates generally to a method and an apparatus for grinding the foremost end of a ferrule integrated with an optical fiber employable for an optical component such as an optical fiber connector or the like to be used for an optical fiber communication circuit system. More particularly, the present invention relates to a method and an apparatus for stepwise grinding the foremost end of a ferrule of the foregoing type at an improved dimensional accuracy with a reduced number of manhours. Further, the present invention relates to a ferrule integrated with an optical fiber wherein the ferrule is produced by employing the method and apparatus constructed according to the present invention.
2. Description of the Related Art
As is well known by any expert in the art, an optical connector has been hitherto extensively used as means for connecting an optical fiber to an exposing optical component in an optical communication circuit network. Usually, the optical connector is produced by way of the steps of inserting an optical fiber through a central hole of a ferrule, fixedly securing the optical fiber to the ferrule with the aid of an adhesive and then grinding and polishing the ferrule together with the optical fiber along their foremost end surfaces.
To reduce loss appearing on the connection surface where the optical fiber is optically connected to the opposing optical component, it has been heretofore required that loss due to reflected light return be minimized. In view of the foregoing requirement, it is advantageously acceptable to grind and polish the foremost end surface of the ferrule while exhibiting a spherical contour having a predetermined radius of curvature. In this connection, a grinding method which creates a ground surface with the ferrule integrated with an optical fiber without light reflection should be considered.
In view of the current circumstance as mentioned above, the inventor has conducted a variety of research and development activities and proposed on various types of grinding methods and grinding apparatuses.
For example, one of the proposals is concerned with a method of grinding the foremost end surface of an optical fiber to exhibit a convex spherical contour wherein a convex spherical surface portion having a predetermined radius of curvature is formed on the lower surface of a circular disc at the central part of the same, an optical fiber is projected downward of the convex spherical surface portion by a predetermined quantity, and the foremost end surface of the optical fiber is rotated about an optical axis of the same while the projected part of the optical fiber is pressed against an abrasive film attached to a soft plate molded of an elastomeric material so as to allow the abrasive film to be warped to assume a concave spherical contour (refer to Japanese Patent Application NO. 91715/1987 filed by the assignee of this invention which is titled "A method of connecting an optical fiber to a certain opponent optical component, an apparatus for grinding an optical fiber to practice the foregoing method and and an apparatus for connecting an optical fiber to a certain opponent optical component").
In addition, another proposal is concerned with an apparatus for simultaneously grinding the foremost end surfaces of a plurality of optical fibers each integrated with an optical fiber with a high grinding quantity to assume a convex spherical contour wherein the ferrules are mounted on a circular disc-shaped jig together with the optical fibers along the circumferential region of the same, a grinding frame is mounted on the central part of the jig so as to adjust the pressure to be imparted to a grinding table having an abrasive film attached thereto, and the grinding frame is then pressed against the grinding table for rubbing the foremost end surfaces of the ferrules with the abrasive film while the jig revolves so as to allow the foremost end surfaces of the ferrules to move along a small circular locus and the grinding table is rotated so as to allow the jig to move along a large circular locus (refer to U.S. Pat. No. 4,831,784 issued May 23, 1989 to the inventor of this invention, which is titled "Polishing apparatus for end surfaces of optical fibers").
According to the latter proposal, the foremost end surfaces of a plurality of ferrules each integrated with an optical fiber can simultaneously be ground at a high efficiency with a high grinding quality.
However, it has been found that the process of turnably displacing the grinding frame has a problem that the quality of the grinding operation varies depending upon on the grinding frame holding position attributable to the foremost end surface of the ferrules, the abrasive film and the grinding direction. To obviate the aforementioned problem, the inventor made a proposal for an improved apparatus for grinding the foremost end surfaces of optical fibers, and U.S. Pat. No. 4,979,334 was issued on Dec. 25, 1990.
According to this proposal, the apparatus is constructed such that the grinding frame is not turnably displaced but the grinding table is rotated so as to allow each ferrule to move along a larger circular locus while the grinding wheel is eccentrically supported. With this construction, the problem associated with the quality of grinding operation which varies dependent on the grinding frame holding position can be solved satisfactorily. In addition, in contrast with the conventional apparatus, there does not arise any malfunction attributable to the displacement of the grinding table as mentioned above. Consequently, a number of ferrules each integrated with an optical fiber can be produced with a high grinding quality.
FIG. 5 is a sectional front view of a grinding apparatus of the foregoing type, particularly illustrating the arrangement of a holder assembly for holding a ferrule integrated with an optical fiber and a grinding table.
A conventional manual grinding operation for grinding the foremost end surface of the ferrule integrated with an optical fiber such as an optical connector or the like will be described below with reference to FIG. 5.
A ferrule 31 having an optical fiber 32 fitted into a central hole thereof is inserted through a fitting hole 34 which is drilled through a circular disc-shaped holder 33 at the central part of the same. A male-threaded portion is formed around a projection 38 extending upward of the holder 33. After the ferrule 31 integrated with the optical fiber 32 (hereinafter referred to simply as a ferrule) is inserted through the fitting hole 34, a flange portion of the ferrule 31 is firmly placed on the upper surface of the projection 38, whereby a reference length H is determined based on the distance through which the ferrule 31 is to be inserted. Then, the ferrule 31 is fixedly mounted on the holder 33 by depressing the flange portion of the ferrule 31 by threadable engagement of a female-threaded portion of an engagement nut 35 with the male-threaded portion of the projection 38.
The reference length H of the ferrule 31 is determined such that the foremost end surface of the ferrule 31 is projected downward of the lower surface of the holder 33 by a quantity of about 0.1 to 0.2 mm.
To grind the foremost end surface of the ferrule 31, an abrasive film 36 is adhesively placed over the upper surface of a base board 37 made of a glass or a synthetic rubber.
Generally, the grinding process is practiced by way of three steps, i.e., a step of coarse grinding with abrasive grains each having a size of about 15 microns, a step of intermediate grinding with abrasive grains each having a size of about 3 microns and a step of finish grinding with grains each having a size of about 0.5 to 1 micron.
The grinding operation is performed by displacing the holder 33 along an arc-shaped locus while the lower surface of the holder 33 is brought into close contact with the working surface of the abrasive film 36.
The first step of coarse grinding is completed when the foremost end surface of the ferrule 31 becomes flush with the lower surface of the holder 33. For this reason, the next step of medium grinding can not be performed any more because the ferrule 31 does not project downward of the lower surface of the holder 33. In other words, the grinding operation can not be performed further unless the holder 33 is replaced with another holder.
To obviate the foregoing difficulty, one set of holders, i.e., three holders each having a different reference length H should be prepared so as to reserve a certain quantity or amount of projection of the ferrule 31 for each grinding operation.
Therefore, to practice the conventional method as mentioned above, it is necessary that after completion of a grinding operation, the present holder be replaced with another holder having a different reference length H which in turn is fitted with the ferrule removed from the present holder, resulting in a complicated grinding process requiring on increased number of manhours.
Especially, in a case in which ten or more ferrules each integrated with an optical fiber are simultaneously mounted on a holder so as to allow the foremost end surfaces of the ferrules to be simultaneously ground, there arises a difficulty in that an error is caused each time the present holder is replaced with another holder, resulting in the quality of the grinding operation being degraded.